The present invention relates to radiation devices and, more particularly, to devices for use with ionizing radiation capable of providing a calibrated reference image with which elements of a detected image may be compared.
As is well known, computer aided tomography (CAT) techniques employ a source of ionizing radiation positioned at one location with respect to a body, and one or more detectors positioned at the opposite side of the body. The source and detectors are moved together in a scanning arc about the body while the outputs of the detectors are stored as a measure of the attenuation of the body at a plurality of positions about the arc. Typically, the arc encompasses a 360-degree circle about the body and attenuation measurements are made at, for example, one-degree intervals.
The above scanning technique produces a matrix of attenuation values for the various angles conventionally processed by computer to yield reconstructed images having image densities related to the attenuation of tissue in at least one transverse slice through the body.
Some applications of computer tomography require more quantitative results than the imaging applications discussed above. For example, U.S. Pat. No. 4,l24,799 discloses an elastic belt containing test bodies strapped about a patient within the field of view of the scanning apparatus. The test bodies contain calibrated concentrations of materials whose presence in the resulting reconstructed image provides a set of known reference values for comparison with the images of nearby tissue to calculate a radiation dosage for later radiation treatment.
The technique of strapping an elastic belt about the patient's body is less than satisfactory for quantitative computer tomography of a human spine. One significant application of such techniques is in determining the chemical content of the spongy bone, also called the trabecular bone, in the human spinal column. Of particular interest is the amount of calcium-containing compounds in the bone. A quantitative measurement of calcium in the spongy bone permits diagnosis and treatment of certain bone-degenerative diseases.
A reference phantom containing graduated concentrations of bone-reference materials for quantitative spinal imaging is disclosed in U.S. Pat. No. 4,233,507, of common assignee with the present application. Other reference phantoms are described in papers "Comparison of Vertebral and Peripheral Mineral Losses in Disuse Osteoporosis in Monkeys"; Cann CE, Genant HK, Young DR; RADIOLOGY, 134:525-529, February 1980; (presented as work-in-progress at the Radiological Society of North America meeting in November 1978); and "Integral Approach to Vertebral Bone Mineral Analysis by X-Ray CT"; Kalendar WA, Suess C, Kotz E; presented as work-in-progress and Scientific Exhibit at Radiological Society of North America meeting in November 1985.
The above cited reference phantoms include rigid plates either embedded in, or disposed parallel to, the supporting table and containing the reference material. In the above patent and papers, the reference material is incorporated into, or laid atop, a planar or transversely curved table top.
The curvature of the human spinal column produces an air gap between the reference phantom and the patient's back. The air gap induces shading and streak artifacts capable of affecting the reference values, thus leading to possible errors in quantifying the results of the measurements. The above patent proposes reducing the size of the gap by raising the knees of the patient to reduce the angle of the pelvis. Even when this is done, however, a substantial curvature, and resulting air gap, remains.
The above papers propose that improved accuracy of determining bone-mineral values may be achieved by coupling the reference phantom to the patient using, for example, an intermediate material substantially filling the air gap and having a response approximating that of water. One material which appears to be satisfactory for this purpose is a bolus, or flexible container, filled with water and positioned between the reference phantom and the back of the patient. The addition of a bolus of water-equivalent material is undesirable since it attenuates the portion of the X-ray beam passing through it by a factor of as much as 3 and increases the statistical error in the measurements due to increased photon noise. Besides a water-filled flexible bag, other coupling materials such as, for example, gels and powders having properties corresponding to those of water, have been used. Such other coupling materials, having properties equivalent to water, share the undesirable results of water itself. .This paper also proposes that edge effects caused by off-focal radiation may be minimized by encapsulating the sample materials in the reference phantom in a near-water-equivalent material such as, for example, an acrylic.